This invention relates generally to the ultrasonic bonding of materials and more particularly to a method and apparatus for optimizing wire bonding, for example, in the manufacture of semiconductor integrated circuits.
In the manufacture of integrated circuits, wire bonding is used to connect the electrical contacts on the semiconductor devices to larger stronger leads on the IC package. Typically, the device contacts are aluminized bond pad areas. A fine gold or aluminum wire must be metallurgically connected to both the bond pad and to the IC package lead in a stress-free loop that can withstand certain mechanical punishment.
In the ultrasonic bonding process, a metallurgical bond is achieved through the proper transmittal of ultrasonic energy under pressure to the bond interface. Consistent and reliable bonding requires optimization of the parameters of energy, clamping force or pressure, time, and bonding pad and IC package condition. The term ultrasonic bonding as used herein includes thermosonic bonding wherein heat is applied during the bonding process. Various combinations of energy amplitude, pressure, time and bonding tool configurations have been used to attempt to achieve consistent high quality bonds. However, the parameters for a particular bonding system are determined in a trial and error manner by adjusting the parameters and inspecting the resultant bonds until consistent bonds having high pull strength have been achieved.
One disadvantage of this method is that, even if optimum settings are achieved at the beginning of a production run, the degradation in the bonding tool after a certain number of bonds, variations in the amplitude and time settings, and variations in the wire or bonding pad materials may cause inconsistent or incomplete bonds. For example, an excessive time setting can cause cratering, or the fracturing of the silicon under the bonding pad, as well as aluminum buildup on the bonding tool necessitating more frequent replacement. Too low a time setting, on the other hand, can cause an incomplete bond which may lead to rejection of the entire IC. This is especially critical in the case of automatic bonders where numerous bonds are made in rapid succession. Many insufficient bonds may go undetected before corrective actions can be taken.